Why Thermal in a Slip? | And the Hazards of Skidding

Experienced soaring pilots usually advise thermalling in a slip. Dick Johnson in the October 2004 issue of Soaring magazine provides an excellent technical explanation for how and why to use this technique.

The bottom line is that a turn requires a combination of pitch, bank, and yaw to maintain a stable configuration. And it requires some compromises to achieve the most efficient combination.

If you thermal fully coordinated, you will need to use top aileron to avoid “falling into the turn”. If you slip using top rudder, you will minimize or even eliminate the top aileron. You are compromising by dragging the fuselage through the air stream, while making the wing cleaner.

It seems like some combination of top rudder and top aileron is often most efficient.

Conversely, turning with ANY amount of skid is VERY inefficient. The MORE you skid, the MORE top aileron you need to avoid falling into the turn.

In this case, you are BOTH dragging the fuselage through the air stream AND using quite a bit of aileron to keep the turn stable. This is very bad!

I notice that many pilots thermal with a heavy inside foot, resulting in a skidding turn. Aside from being more spin prone, even a minor skid will be very draggy!

Next, skidding into a turn (leading with the rudder) makes the glider especially spin prone. Initially you rudder into the turn, followed by feeding in back stick and opposite aileron to stabilize the turn. This serves to load up the glider while putting in control inputs that aggravate stalling/entering a spin.

This is especially dangerous in the pattern. Many pilots have a tendency to over-rudder their turns while on approach.

Here is a scenario that can easily result in a stall/spin. A low and slow base leg, followed by an over-ruddered, button-hooking turn.

As you start turning, you use too much rudder. This is followed by pulling back to get the turn established. And finally, feeding in opposite aileron to keep the glider from falling into the turn. This results in the glider departing into an unrecoverable spin.

5 Replies to “Why Thermal in a Slip? | And the Hazards of Skidding”

  1. While pilots can be taught to avoid this, I think a useful thought which is not taught is that you don’t have to do a normal circuit if you’re low. Turn onto base leg half way along downwind and set yourself up for a landing half way down the field. We don’t have to treat the airfield as though it has some sort of barrier around it that we can’t cross except from the very end on finals: it’s just a flat surface to land on – if we land well past the painted numbers but safely, we’ve succeeded.


    1. Great video!

      Early in learning to teach, I made the mistake of teaching spin training late in the process.
      The scenario you describe was also prevalent for students about half way to solo as we taught ‘standard pattern’ profiles, and high and low approaches to develop perspectives for judgement. We noticed some students would compensate, or sacrifice bank angle to ‘feel more comfortable’ when low. Making shallow banks on the low approach as they lined up the runway. It appeared instinctively, but they would chose, or decide, to shallow the bank when low. ‘Putting the wing down’ apparently added some fear. This was all mitigated in spin training, and a view from outside the cockpit, ensuring wing tip to ground clearances. Since then, we moved spin training earlier, much earlier, into the training cycle. To instill good habits, risk mitigation, and confidence, I suggest the spin training be moved to shortly after stall training. ‘Mild’ ones, to install the laws of primacy, and proper reflexes, not instill fear, bit confidence.

      A lower altitude base leg mades some students worried about a steep turn. Shallow bank on turn to final, a rudder into the turn to ‘help it around’, and the rest is covered in your video. Opposite aileron, back stick, wham. Not good. And no button hook needed, just a flat, differed turn.

      Great job Daniel!!

      Dave, Sr.


  2. Hi Daniel. I’m trying to find the Dick Johnson article on thermaling in a slip that you mention above, but I’m not finding it in July 1997 magazine issue. You sure about that month/year?


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